Static magnetic field metal detector

ABSTRACT

A static magnetic field detector senses the passage of ferrous or magnetic objects, employing a balanced pick up coil configuration providing substantial self-cancellation of spurious background signals. The pick up coils are disposed in a uniform static magnetic field and are cooperatively effective to sense the presence of such objects with respect to an extended passage way by reacting to the field distortion caused by the object to produce an alarm or control signal. Further reduction of the effects of spurious background signals is afforded by novel means for processing the signals generated within the detector coils providing filtering and threshold functions. An adaptive threshold control circuit provides automatic adjustment of the threshold level of object detection.

United States atent Eennett, 51". et al.

[ Sept. 11, 1973 STATIC MAGNETIC FIELD METAL DETECTOR 3,675,660 7/1972 Girodat 130/27 JT Primary Examiner-Louis G. Mancene Assistant ExaminerJ. A. Oliff At-torneyHoward P. Terry [57] ABSTRACT A static magnetic field detector senses the passage of ferrous or magnetic objects, employing a balanced pick up coil configuration providing substantial selfcancellation of spurious background signals. The pick up coils are disposed in a uniform static magnetic field and are cooperatively effective to sense the presence of such objects with respect to an extended passage way by reacting to the field distortion caused by the object to produce an alarm or. control signal. Further reduction of the effects of spurious background signals is afforded by novel means for processing the signals generated within the detector coils providing filtering and threshold functions. An adaptive threshold control circuit provides automatic adjustment of the threshold level of object detection.

12 Claims, 9 Drawing Figures PATENTEI] SEP] 1 975 SHEET 1 or 4 PATENTEDSEN I973 3-, 7 57 501- v SHEET 2 0F 4 46 ALARM f CH.2 T

47 50 6 /p (DQ0- 43/ 7 53 CONTROL L Q0: Q3 PROCESSOR 45/ 56 T 54W T 180 FROM DRIVE MOTOR SOURCE- OF I o IRECT CURRENT I STATIC MAGNETIC FIELD METAL DETECTOR BACKGROUND OF THE INVENTION 1. Field of the Invention The invention pertains to apparatus for detecting the presence or passage of objects composed at least in part of ferrous materials such as iron or steel and more particularly relates to improved detection devices of the foregoing type exhibiting high immunity to spurious background or noise signals and minimum susceptibility to generation of false alarms or other undesired reactions.

2. Description of the Prior Art Generally, prior art devices for detecting the presence of metallic ferrous or other objects have one or more disadvantages rendering them of little value in certain applications, especially in the reliable protection of expensive machinery from tramp metal, for instance. Such prior art bridge and other devices often operate with excitation frequencies as high as to 100 KHz and are quite susceptible to the presence of moisture, vegetation, the operator, or the like. They therefore impose on the operator the serious burden of continuous monitoring and adjustment of the calibration of the apparatus. Even at relatively lower excitation frequencies such detectors are adversely affected to a significant extent by moisture conditions, thereby rendering them unsuitable for use where high accuracy, reliability, and freedom from generation of false alarms is demanded. Additionally, such inductive loop detectors are highly susceptible to noise signals generated in moving parts of machinery, such as metal joints in conveyor belts and gear teeth or other similar projecting parts of rotating machine elements. Such machinery elements produce either false alarms in prior art detectors, or require unsatisfactory reduction of sensitivity. For example, little success has been shown in the prior art in the line of providing equipment for detecting tramp metal entering expensive machines such as farm tools because of the difficulty in separating the signals produced by the tramp metal from the high level of spurious noise produced by moving machine parts of the farm tool.

SUMMARY OF THE INVENTION The present invention overcomes the aforementioned limitations of prior art metal detectors by the provision of apparatus constructed in such a manner as to be free of adverse effects of moisture and immune to spurious noise signals produced by the proximate moving parts of a protected or monitored machine. The problems of the prior art are overcome by employing specially shaped, overlapping, balanced signal pick up coils disposed in a substantially uniform static magnetic field in a novel configuration for providing substantial cancellation of spurious background signals. The signal pick up coils are cooperatively effective to sense the passage of ferrous or magnetic objects, whether they represent desired or undesired objects. Sensing is achieved by differential signals induced in the pick up coils caused by the distortion of the magnetic field by the object. Further reduction of background signal level is effected by filtering and threshold level control functions performed in the invention when the pick up coil outputs are processed and combined for actuation of an alarm or control device. The threshold level may be set manually or automatically to an operating level minimizing false reactions. Use of the invention in a variety of applications such as protection of machinery from tramp metal, monitoring of manufacturing processes, and in security systems is contemplated. For example, the apparatus may be used to detect ferrous tramp metal in wood pulp or in ore on a conveyor belt, thus preventing damage to manufacturing machines receiving such wood pulp or ore. On the other hand, it may be used to recognize desireable ferrous metal parts in certain manufacturing processes. For illustrative purposes, the invention will be described in this disclosure in use for protecting a forage harvesting machine from ingestion of ferrous tramp metal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary elevation view, in part in cross section, of a forage harvesting machine illustrating one application of the invention.

FIG. 2 is a view of a part of the machine of FIG. 1 showing the location of the novel pick up coils relative to a floor of the machine and showing electrical connections between the pick up coils and signal processing and utilization apparatus.

FIG. 3 is a perspective view somewhat similar to that of FIG. 2, but showing the location of a magnetic field excitation coil used in the invention.

FIG. 4 is a view similar to FIG. 3 showing an alternative magnetic field excitation system.

FIG. 5 is a circuit diagram of the pick up and excitation coils of FIGS. 2 and 3.

FIG. 6 is a circuit diagram of the pick up and excitation means of FIGS. 2 and 4.

FIG. 7 is a diagram useful in explaining the operation of the pick up coils of FIGS. 5 and 6.

FIG. 8 is a circuit diagram showing electrical connections between the pick up coils and the elements of the signal processor of FIG. 2.

FIG. 9 is a circuit diagram of an alternative circuitfor use in the system of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates an improved forage harvesting machine of one of the kinds in which the present invention may be employed. It will, however, become apparent that the invention may be employed for the protection of other types of farm and general utility machinery, and that it has other monitoring and alarm actuating applications aside from protecting machinery from damage caused by the ingestion of objects of ferrous or magnetic materials. The particular forage harvester of FIG. 1 is similar to that described by T. W. Waldrop and H. C. Eberly in the U.S. Pat. No. 3,523,411 for a Forage Harvester Device," issued Aug. 11, 1970 and assigned tothe Sperry Rand Corporation.

The forage harvester of FIG. 1 includes a wheelmounted frame or housing construction 10 for supporting a forage cutter-head l8 and a crop pick up unit 11 that ia pivotally mounted at 12 for support in part from the basic vehicle frame work 13 by which the harvester may be towed by a suitable tractor (not shown). The harvester further includes at least one set of in-feed conveyor rolls 14 and 15, mounted for rotation about substantially parallel axes 14a, 15a and journaled in housing 10. A forage cutter head assembly 18 is mounted within housing 10 to the rear of rolls 14, 15 for rotation on shaft 18a and for receiving forage ejected from rolls 14, 15. A discharge spout 20 extends vertically and then horizontally from the rearward portion of housing 10.

The crop pick-up unit 11 comprises a conventional rotatable reel 21 on a shaft 2 1a having plural radially projecting, laterally spaced pick-up tines or fingers, such as tine 22. A plurality of conventional laterally spaced stripping or doffing members (not shown), between which the tines 22 pass as reel 21 rotates the tines into housing 11, lifts the crop into the region of influence of a conventional rotatable auger 24. Auger 24 has oppositely directed helical flights for directing the crop material from both sides of the auger input to- .ward its center and therefore into the input of the infeed conveyor rolls 14, 15. Auger 24 is mounted for rotation on shaft 24a. Auger 24 may be of the general type described by L. M. Halls and H. G. McCarthy in the US. Pat. No. 3,324,639 for Auger Header and Crop Conditioner," issued June 13, I967 and assigned to the Sperry Rand Corporation.

Although a crop pick-up 11 is shown in FIG. 1, it is apparent that a sickle bar or row-crop attachment could also be employed to deliver a crop .into in-feed rolls 14, 15. In the unit shown, harvested crop material to be comminuted is picked up by the reel 21 and auger 24 of crop pick-up unit 11 and is delivered to the infeed rolls l4, 15. From the rolls 14, 15, the crop material is metered in a mat-like form to cutter head 18, where it is comminuted by a rotating array of knives such as knife 18b and is finally discharged through discharge spout 20 into a suitable receptacle, such as a truck or wagon (not shown).

Upon consideration of the above-mentioned Waldrop et al. US. Pat. No. 3,523,41 1, it will be clear that, for the sake of simplifying the drawing of FIG. 1, the vehicle wheel companion to wheel 25 has not been shown in the drawing of FIG. 1. Also, a plurality of drive belts or other drive means by virtue of which elements including reel 21, auger 24, and rolls 14 and 15 are driven with respect to the power source driving shaft 18a and therefore with respect to cutter head 18 are absent for the sake of simplifying FIG. 1. Such devices are fully described in the above-mentioned Waldrop et a] patent and elsewhere and provide means for driving the various rotatable elements of appropriate rotational speeds in the directions indicated by arcuate arrows A, B, C, D, and E. When operated in such fashion, the flow of crop material is over reel 21 and into auger 24 along the path of arrows M and N and along path intorolls 14, 15. Ejected from rolls 14, along path P into cutter head 18, the forage then follows paths 0 and R between the rotating cutter head 18 and guide 26, and finally moves upward along path S within pipe 27 and out of spout along path T. In traversing paths N and 0 toward rolls 14, 15, the forage material passes over a floor plate 30 indicated in a general fash-' ion by the dotted lines in FIG. 1. It will be understood that the dimensions and proportions shown in FIG. 1 and in the several figures yet to be discussed are selected for the sake of making the drawings clear, and are therefore not necessarily dimensions or proportions which would be used in actual practice. It will be understood that, in typical use of the invention on farm machines, the detector may be located in a variety of places such as at the front of the vehicle, or on the tractor pulling the vehicle.

During various operations using farm tools, such as sowing, planting, mowing, raking or the like, it is possible for machine parts made of iron or other magnetic materials to break off or to simply drop off the farm machine being used without knowledge of the operator and to be lost in the field. Typically, these pieces of tramp metal are composed of magnetic materials such as iron, and include bolts, pins, rake teeth, idler pulleys, chain links, mower sickle teeth, hand tools, and the like. During use of a forage harvester such as that of FIG. 1 for harvesting sorghum, alfalfa, or other types of fodder, such pieces of tramp metal can be picked up by the harvester pick-up unit 11 and may pass into the environs of the cutter head 18, causing extensive damage.

To prevent such damage during the critical time of forage harvesting, it is desired to place a tramp metal presence detector in the path M-N-O-P that the crop travels in moving toward cutter head 18. The tramp metal detector is placed in such a location that sufficient time is allowed in which manually or automatically to stop at least rolls l4, 15 before the trampmetal part reaches cutter head 18. It will be apparent that other or alternative moving parts of the forage harvester of FIG. 1 or of other monitored equipment may be automatically caused to stop, such as cutter head 18 or even all of the rotatable parts of the farm machine. The towing tractor may also readily be stopped automatically or by the operator in response to a detector actuated alarm.

As is seen in FIG. 2, the tramp metal sensor signal pick-up coils are mounted in close association with each other and with floor plate 30 which forms a part of the structure of the cutter head housing 10, being bolted to the hay header. 11 in front of the aperture leading to rolls 14, 15 by using conventional bolt holes such as holes 61,62. Sensor coils 40 and 41 are preferably multiple turn coils placed in a unidirectional magnetic field generated by means'yet to be discussed in further detail in connection with FIGS. 3 and 4. A desired magnetic field line of flux is represented by the vertical plane loop 42; only one such loop is drawn for convenience and it will be understood that loop 42 represents one loop of a uniform unidirectional magnetic field extending all along signal pick-up coils 4 and 41. In the presence of such a unidirectional field, an output voltage is generated by one or both coils 40, 41 when an object of ferrous material such as pin 60 passes into the region of the magnetic field. It will be understood that signal pick-up coils 40, 41 for sensing any magnetic field change caused by pin 60 are located in the same region as the source of unidirectional magnetic field so as substantially to reduce the noise signals coupled to signal pick-up coils 40,41 caused by rotation of elements such as feed rolls 14, 15, auger 24, and the like. I In the presence of a ferrous object 60, a corresponding monitoring alarm signal is generated in coil 40, in coil 41, or in both coils and is received on leads 43 and 44 leading to signal processor 45. Any output of signal processor 45 appearing on lead 53 may be coupled through manual selector switch 46 to an alarm 48, which may be a conventional visual or audible alarm, including a latching alarm resettable by manual operation of a conventional re-set button 49. In addition, or alternatively, the operator may use manual selector switch 47 to operate a conventional servo or other control 50. With manual selector switch 51 closed, the operator may cause control 50 to operate a conventional rapid acting clutch 53 placed between the harvester drive motor and shaft 18a. He may, in addition, close switch 52 to cause control 50 to operate a conventional rapid-acting brake 54 in order faster to stop rotation of shaft 18a after the declutching event. Shaft 18a may, for example, be the shaft directly driving the cutter head 18 of FIG. 1 or other of the rotating elements of the figure, such as shaft 14a.

FIG. 3 illustrates one method of supplying the unidirectional magnetic feld represented by the flux line 42 of FIG. 2. In FIG. 3, the signal pick-up coils 40, 41 are not shown for permitting clarity in the drawing, and it will be understood that the field excitation means is a multiple-turn elongate coil 72 which may substantially encompass or be entwined with coils 40, 41, which latter coils are also enclosed in potting material along with coil 72 in the elongate steel box 76 holding the several coils adjacent slot 71 and generally near the bottom of floor plate 30. Channel or slot 71, which may be closed by a plastic material, is provided to permit the desired magnetic field to pass from coil 72 through the slot 71 into the region immediately above the steel floor plate 30 and to permit perturbations in the field to couple to coils 40, 41. The desired magnetic field is supplied to coil 72 via lead 73 when direct current source 75 is connected to coil 72 by the closure of switch 74.

FIG. 4 represents a preferred method of providing the desired substantially uniform unidirectional magnetic field represented by flux loop 42 of FIG. 1. In FIG. 4, the signal pick-up coils 40,41 are again not shown for allowing clarity in the drawing. It will be understood that the magnetic field excitation means consists of a multiciplicity of generally U-shaped magnets 80, 81, 82, 83, and 84 about which signal pick-up coils 40, 41 may be placed, which magnets 80 to 84 and coils 40,41 may be enclosed in coil potting material in an elongate aluminum box 85 holding the magnet array adjacent the bottom of floor plate 30, which floor plate may be constructed of aluminum or other nonmagnetic material. It will be observed that alternate magnets 80, 82, and 84 are of substantially equal length, and that the intervening magnets 81 and 83 are each of equal length and may be shorter than magnet 80, for example. In the figure, floor plate 30 is cut away at its right side and the box 85 and its potting material are also absent in the view so that the typical magnet 84 may be directly viewed. It will be noted by those skilled in the art that the magnets 80 to 84 of FIG. 4 may be used with the slotted (71) steel floor plate 30 of FIG. 3. Likewise, the coil 72 of FIG. 3 may readily be used beneath the non-slotted aluminum floor plate 30 of FIG. 4.

The relation of the excitation coil 72 and of pick-up coils 40, 41 may be further illustrated by considering FIG. 5 along with FIG. 2. In FIG. 5, coil 40 is seen to be generally rectangular in shape, but is twisted into something like a figure eightby the mid-plane cross over at location 90 into two equal-area coil sections 40a and 40b. Similarly, coil 41 is seen to be of generally rectangular shape, but is twisted into a second figure eight by virtue of the mid-plane cross over'91 into two equal-area coil sections 41a and 41b. While coils 40 and 41 appear to be laterally off set in the schematic drawing of FIG. 5, they are preferably symmetrically related and intertwined and may be woven around a single coil form.

The several space quadrants Q Q Q and Q of FIG. 5 extend in total across the aperture through which the crop feeds from auger 24 into rolls 14, 15. It is noted that coil sector 40a lies in quadrants Q, and Q coil sector 41a lies in quadrants Q; and Q3, coil sector 40b also lies in quadrants Q, and Q3, and coil sector 41b lies in quadrants Q and 0,. Cross overs at 90 and 91 lie at mid-points in the central quadrants Q and Q FIG. 6 shows a similar configuration of signal pick-up coils when employed, as in FIG. 4, with permanent magnets 80 to 84. The respective lengths and separations between the similarly poled magnets 80 to 84 permit multi-turn coil 40 to be threaded between the upright poles 95 and 96 of magnet 81 and the similar poles of magnet 82. The cross over at location 90 is performed and the coil 40 is then threaded in reversed direction between the poles of magnets 83 and 82 again to the cross over location 90. Similarly, multi-turn coil 41 is threaded between the poles of magnets 81 and 82, through cross over location 91 back through magnets 84 and 83, and through cross over location 91. Coils 40 and 41 may be encapsulated along with the similarly poled magnets 80 to 84 within box 85 of FIG. 4. It will be observed that the same general geometry with re spect to quadrants Q Q Q and Q, is employed in both FIGS. 5 and 6.

Operation of the sensor and associated system will be further apparent from FIG. 7, which shows only sensor pick-up coil 40 with the mid-plane twist at location 90 forming coil sectors 40a and 40b encompassing respective regions A and B. As has been observed, signal pickup coil 40 has roughly the form of a figure eight symmetric about mid-plane 100 and is a multiple turn coil in order to raise the signal generated by object 60 to an appreciable level. In representative apparatus, the pick-up coil may have 1,000 turns for use with a 12 volt battery driving a 100 turn exciter coil for producing a field on the order of 100 gauss at the floor plate.

The static magnetic field threading signal pick-up coil 40 produces a total flux 4),, passing through region A and a total flux (it passing through region B of signal pick-up coil 40. These two fluxes may readily be made substantially equal by use of appropriate precision in manufacture of coil 40. With a substantially constant amplitude magnetic field along the total length of quadrants Q Q Q and Q,,, the amplitudes of and g may be made equal and will have the same sign. A finite voltage V will be produced at the output 43 of coil 40 only when there is a change of flux passing through coil 4'0:- V N .4/dt da/dt] where N is the number of turns of wire in coil 40. A finite differential pulse is produced when object '60 passes above coil 40 in an unsymmetric position with respect to cross over location and mid-plane and the pulse is passed as a useful alarm signal via leads 43 to signal processor 45 of FIG. 1. Should bolt 60 pass symmetrically over the cross over location 90 in plane of symmetry 100 of coil 40, it will disturb coil sectors 40a and 40b substantially equally with a resultant zero output on leads 43.

However, such a property is a'distinct benefit in the invention, since various parts of the forage harvester have effectively a considerable degree of symmetry about a mid-plane cross over such as that at location 90. For example, the teeth of feed rolls 14, 15, the feed rolls themselves, the auger 24, other rotating parts of the machine and, finally, the main frame and housings of the harvester are effectively symmetric in substantial degree about location 90. Thus, rotation of moving parts of the harvester and vibration of the frame and housings thereof produce, in large measure, flux disturbances in coil 40 which are self-cancelling.

To obviate the problem posed by the blind location in coil 40 when object 60 passes above mid-plane cross over location 90, it is seen in FIGS. 2, 5, 6, and 8 that a pair of similar pick-up coils 40 and 41 is employed. In FIG. 8, it will be apparent that coils 40 and 41 have similarly effective degrees of symmetry with respect to the various magnetic field noise sources present in the harvester. n the other hand, coil 41 is fully sensitive to an object 60 passing in the blind mid-point location 90 of coil 40. Likewise, coil 40 is fully sensitive to an object 60 passing in the blind mid-point location 91 of coil 41. Thus an output alarm signal will appear either on leads 43 or on leads 44 for an object 60 passing into the sensor at any position within quadrants Q Q or 0,, including the cross over locations 90 and 91.

As seen in FIG. 8, any output signals signifying the presence of a tramp metal object 60 are coupled on leads 43 to channel CH1 of the signal processor 45; likewise, any such signals on leads 44 are supplied to channel CH2 of processor 45. Channels CH1 and CH2 may be duplicates. For example, channel CH1 comprises a pass band filter or pass band amplifier 120 in series connection with a variable attenuator 121 and a bipolar threshold circuit 124. Similarly, channel CH2 has, in series relation, a band pass filter or amplifier 110, a variable attenuator'lll, and a bipolar threshold circuit 114.

For example, the output signal appearing on leads 43 is passed in channel CH1 to band pass circuit 120, which has a band width, for example, of approximately Hz with a lower cut off frequency of about 0.5 Hz. This pass band enables circuit 120 to pass the frequency spectrum of typical tramp metal objects. The pass band in a typical example rolls off at the low frequency end at substantially 12 dB per octave and at the high frequency end by substantially 18 dB per octave. The pass band characteristics of circuit 120 thus provide an additional reduction in the relatively higher frequency noise signals induced by motion of the various parts of the protected machine, such as the teeth of upper and lower rolls 14, 15. Further, circuit 120' provides a reduction by substantially 60 dB for 60 Hz signals that may be induced by the proximity of electrical power distribution lines. Amplification provided by circuit 120 may be used to raise the signal incoming on leads 43 from, for example, tens of microvolts to hundreds of millivolts.

The output of pass band circuit 120 is passed to the signal balancing attenuator 121, manually adjustable by virtue of the presence of mechanical link 122 and knob 123. From attenuator 121, the signal is supplied to a conventional bipolar threshold detector circuit 124. If the signal flowing into detector 124 is greater than a predetermined positive threshold value or is less than a predetermined negative value, a finite output signal is yielded of polarity independent of the input signal polarity. The similar elements 110, 111 and 114 of channel CH2 of processor 45 are like those of channel CH1 and operate in similar manner. Thus, whether a signal appears on input leads 43 or 44, a corresponding command signal will flow from detector 114 or from detector 124 to the conventional OR circuit 115.

In the presence of either or both such signals, OR circuit yields an output which may selectively be applied to operate alarm 48 or to operate control device 50, as previously described.

An automatic or adaptive threshold control for performing the functions of manual attenuators 111, 121 in FIG. 8 is shown in FIG. 9, which figure illustrates an alternative circuit for use in channels CH1 and CH2 of FIG. 8. The circuit of FIG. 9, for instance, may be considered in use in channel CH2, being connected to input leads 43 and to terminal 150 and thus replacing elements 120, 121, and 124.

In FIG. 9, the input signal induced by a tramp metal object 60 is supplied to a band pass amplifier 151 similar to band pass circuits 110 and of FIG. 8. The filtered and amplified output of circuit 151 is then coupled to full wave rectifier 152. The rectified output of circuit 152 is supplied by branching lead 153 to threshold detector 155 and to low pass amplifier 154. The output of low pass amplifier 154 is supplied via lead 156 to control the operating point of the conventional threshold detector 155. The output of detector 155 is supplied via lead to OR circuit 115. A circuit similar to that of FIG. 9 is also used to replace the channel CH1 of circuit of FIG. 8.

In operation, the pass band circuit 151 serves, as in the instance of circuits 110 and 120, to eliminate a large part of the noise spectrum present, such as that brought about by the proximity of rotating machinery elements, and to raise the amplitude of the useful information-bearing signal. The use of the full wave rectifier 152 eliminates the requirement for use of the bipolar threshold detectors 114, 124 of FIG. 8, since the positive and negative signals output from amplifier 151 are converted by rectifier 152 into positive signals.

To achieve adaptive or automatic threshold control, the output of full wave rectifier 152 is coupled by lead 153 to low pass filter amplifier 154. Amplifier 154 produces essentially a unidirectional current output substantially proportional in amplitude to the energy continuously present as noise signals. Thus, the operating point of the conventional threshold detector is automatically set by a reference signal at a level just above the level of averaged noise. Only alarm signals exceeding that level are passed via terminal 150 to OR circuit 115. The adaptive threshold feature of the invention is seen to provide an automatically adjustable threshold that varies depending upon the noise environment of any particular situation, thus automatically setting the threshold level to maximize the probability of detection of a metal object 60 while minimizing the probability'of a false alarm.

in an alternative form of the invention, a peak detector is inserted between conductor 153 and low pass amplifier 154. With peak detector 170 not present, the. derived threshold level is proportional to the average value of the full wave rectified noise signal. With peak detector 170 in the circuit, the threshold value is proportional to the maximum peak value of the full wave rectified noise signal. Peak detector 170 is a conventional circuit, preferably adjusted so that it has a negligible attack time and a decay time which may be adjusted to be about ten times the period of the fundamental noise frequency present. The output of peak detector 170 is filtered and amplified, as before, to provide a threshold level that is proportional to the maximum peak value of the periodic noise signal.

It will be apparent to those skilled in the art that the magnetic field excitation means and the signal pick-up coils may be adapted to various configurations for various applications. For example, more than two pick-up coils such as coils 40 and 41 may be employed in an elongated structure. Further, the coils 40 and 41 need not lie in a continuous flat plane, but may be bent into a broad V-shaped configuration about a transverse line half way between cross overs 90 and 91 (FIG. In such a manner, the detector system may be designed closely to fit a Vshaped or U-shaped conveyor system or many other types of substantially symmetric elements. Coils 40 and 41 and the selected magnetic field excitation elements need not lie in a horizontal plane, but may be placed in a vertical wall, for instance, in a relatively narrow passage way for detecting movement of metal parts such as fire arms or stolen goods in the pockets or baggage of personnel or other persons.

As has been observed, the invention lends itself to use in detecting undesired ferrous metal to prevent damage to machines. Furthermore, the invention may be used to protect a product from damage due to the inclusion of ferrous material where it would not directly harm the manufacturing equipment itself. It is highly desirable, for example, to prevent iron parts from entering a furnace for making very pure glass or other ceramic materials such as those requiring very low loss, high dielectric properties. In addition, the apparatus is useful, for instance, for determining the desired presence of small ferrous objects on a conveyor belt for operation of an alarm or for direct actuation of machinery such as packaging machinery. It is also apparent that relative motion between a fixed metal object and the moving detector may be sensed according to the invention.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

We claim:

1. In a forage crop harvester having rotatable machine elements including crop processing elements susceptible to damage by a tramp metal part the combination comprising:

means for generating a substantially uniform unidirectional magnetic field on said harvester, multiple conductor magnetic field pick up coil means disposed in said unidirectional magnetic field having output conductor means, said coil means having first and second sections symmetrically located about a mid-plane of said coil means,

cross over conductor means for connecting opposite conductors of said first and second sections at said mid-plane for providing a detection signal on said output conductor means in the presence of said tramp metal parts adjacent said first or second sections,

threshold detector means coupled to said output conductor means, and

utilization means responsive to said threshold means whereby said tramp metal part is detected substantially without interference due to spurious varying magnetic fields generated by said rotatable machine elements.

2. Apparatus as described in claim 1 wherein:

said harvester has a substantial degree of symmetry about a vertical plane, and

said mid-plane is disposed sufficiently close to said vertical plane in relation to said rotatable machine elements to provide substantial cancellation of spurious varying magnetic fields generated by said rotatable machine elements.

3. Apparatus as described in claim 1 additionally including band pass circuit means disposed in series circuit relation between said coil and said threshold detec' tor means.

4. Apparatus as described in claim 3 including in series circuit relation between said coil means and said threshold detector means:

band pass circuit means coupled to said output conductor means, and

attenuator means responsive to said band pass circuit means.

5. Apparatus as described in claim 3 including in series circuit relation between said coil means and said threshold detector means:

band pass circuit means coupled to said output conductor means, and I i full wave rectifier means responsive to said bandpass circuit means. 6. Apparatus as described in claim 5 additionally comprising: v

low pass amplifier means responsive to said rectifier means for generating a control voltage, and

means for supplying said control voltage to said threshold means for adjustably determining the threshold level of said threshold means.

7; In a forage crop harvester of the kind having rotat able machine elements including crop processing elements susceptible to damage by a tramp metal part, the combination comprising:

means for generating a substantially uniform magnetic field in a space within said harvester through which said crop passes into said crop processing elements,

first and second multiple conductor magnetic field pick up coil means disposed in stationary relation in said unidirectional magnetic field and having respective first and second output conductor means,

each of said coil means having first and second sections symmetrically located with respect to first and second mid-planes of said respective coil means,

first cross over conductor means for connecting opposite conductors of said first and second sections of said first coil means at said first mid-plane for providing a first detection signal on said first output conductor means in the presence of said tramp metal part adjacent said first or second section of said first coil means, and

second cross over conductor means for connecting opposite conductors of said first and second sections of said second coil means at said second midplane for providing a second detection signal on said second output conductor means in the presence of said tramp metal part adjacent said first or second section of said second coil means,

said first and second coil means being off set longitu-- dinally one relative to the other so that said first cross over means lies proximate said first section of said second coil means and said second cross over means lies proximate said second section of said first coil means for the purpose of providing output signals at said first or second output conductor means in the presence of said tramp metal part while affording substantial cancellation of spurious magnetic fields induced by said rotatable machine elements. 8. Apparatus as described in claim 7 additionally comprising:

first and second signal processing circuit means respectively responsive to signals at said first and second output conductor means, each said signal processing circuit means comprising in series circuit relation: band pass circuit means, and threshold detector means, OR circuit means responsive to said first and second signal processing means, and

utilization means responsive to said OR circuit means.

9. Apparatus as described in claim 8 further comprising adjustable attenuator means interposed in series circuit relation between said band pass circuit means and said threshold detector means.

10. Apparatus as described in claim 9 further comprising full wave rectifier means interposed in series circuit relation between said band pass circuit means and said threshold detector means.

11. Apparatus as described in claim 8 wherein said utilization means comprises alarm means.

12. Apparatus as described in claim 8 wherein said utilization means comprises control means for stopping said rotatable machine elements subject to damage by said tramp metal part.

a a n: a ti 

1. In a forage crop harvester having rotatable machine elements including crop processing elements susceptible to damage by a tramp metal part the combination comprising: means for generating a substantially uniform unidirectional magnetic field on said harvester, multiple conductor magnetic field pick up coil means disposed in said unidirectional magnetic field having output conductor means, said coil means having first and second sections symmetrically located about a mid-plane of said coil means, cross over conductor means for connecting opposite conductors of said first and second sections at said mid-plane for providing a detection signal on said output conductor means in the presence of said tramp metal parts adjacent said first or second sections, threshold detector means coupled to said output conductor means, and utilization means responsive to said threshold means whereby said tramp metal part is detected substantially without interference due to spurious varying magnetic fields generated by said rotatable machine elements.
 2. Apparatus as described in claim 1 wherein: said harvester has a substantial degree of symmetry about a vertical plane, and said mid-plane is disposed sufficiently close to said vertical plane in relation to said rotatable machine elements to provide substantial cancellation of spurious varying magnetic fields generated by said rotatable machine elements.
 3. Apparatus as described in claim 1 additionally including band pass circuit means disposed in series circuit relation between said coil and said threshold detector means.
 4. Apparatus as described in claim 3 including in series circuit relation between said coil means and said threshold detector means: band pass circuit means coupled to said output conductor means, and attenuator means responsive to said band pass circuit means.
 5. Apparatus as described in claim 3 including in series circuit relation between said coil means and said threshold detector means: band pass circuit means coupled to said output conductor means, and full wave rectifier means responsive to said band pass circuit means.
 6. Apparatus as described in claim 5 additionally comprising: low pass amplifier means responsive to said rectifier means for generating a control voltage, and means for supplying said control voltage to said threshold means for adjustably determining the threshold level of said threshold means.
 7. In a forage crop harvester of the kind having rotatable machine elements including crop processing elements susceptible to damage by a tramp metal part, the combination comprising: means for generating a substantially uniform magnetic field in a space within said harvester through which said crop passes into said crop processing elements, first and second multiple conductor magnetic field pick up coil means disposed in stationary relation in said unidirectional magnetic field and having respective first and second output conductor means, each of said coil means having first and second sections symmetrically located with respect to first and second mid-planes of said respective coil means, first cross over conductor means for connecting opposite conductors of said first and second sections of said first coil means at said first mid-plane for providing a first detection signal on said first output conductor means in the presence of said tramp metal part adjacent said first or second section of said first coil means, and second cross over conductor means for connecting opposite conductors of said first and second sections of said second coil means at said second mid-plane for providing a second detection signal on said Second output conductor means in the presence of said tramp metal part adjacent said first or second section of said second coil means, said first and second coil means being off set longitudinally one relative to the other so that said first cross over means lies proximate said first section of said second coil means and said second cross over means lies proximate said second section of said first coil means for the purpose of providing output signals at said first or second output conductor means in the presence of said tramp metal part while affording substantial cancellation of spurious magnetic fields induced by said rotatable machine elements.
 8. Apparatus as described in claim 7 additionally comprising: first and second signal processing circuit means respectively responsive to signals at said first and second output conductor means, each said signal processing circuit means comprising in series circuit relation: band pass circuit means, and threshold detector means, OR circuit means responsive to said first and second signal processing means, and utilization means responsive to said OR circuit means.
 9. Apparatus as described in claim 8 further comprising adjustable attenuator means interposed in series circuit relation between said band pass circuit means and said threshold detector means.
 10. Apparatus as described in claim 9 further comprising full wave rectifier means interposed in series circuit relation between said band pass circuit means and said threshold detector means.
 11. Apparatus as described in claim 8 wherein said utilization means comprises alarm means.
 12. Apparatus as described in claim 8 wherein said utilization means comprises control means for stopping said rotatable machine elements subject to damage by said tramp metal part. 